                1. Field of the Invention        
This invention relates to optical code readers. In particular, this invention relates to a system and method for reducing motion sensitivity in imager-based optical code readers.                2. Description of the Related Art        
Optical codes are patterns made up of image areas having different light reflective or light emissive properties, which are typically assembled in accordance with a priori rules. The term “barcode” is sometimes used to describe certain kinds of optical codes. The optical properties and patterns of optical codes are selected to distinguish them in appearance from the background environments in which they are used. Devices for identifying or extracting data from optical codes are sometimes referred to as “optical code readers” of which barcode scanners are one type.
Optical code readers are used in either fixed or portable installations in many diverse environments such as in stores for check-out services, in manufacturing locations for work flow and inventory control, and in transport vehicles for tracking package handling. The optical code can be used as a rapid, generalized means of data entry, for example, by reading a target barcode from a printed listing of many barcodes. In some uses, the optical code reader is connected to a portable data processing device or a data collection and transmission device. Frequently the optical code reader includes a handheld sensor which is manually directed at a target code.
An example of a conventional optical code is a one-dimensional barcode symbol. The barcode is a pattern of variable-width rectangular bars separated by fixed or variable width spaces. The bars and spaces have different light reflecting characteristics. One example of a one dimensional barcode is the UPC/EAN code used to identify, for example, product inventory. An example of a two-dimensional or stacked barcode is the PDF417 barcode. A description of PDF417 barcode and techniques for decoding it are disclosed in U.S. Pat. No. 5,635,697 to Shellhammer et al., and assigned to Symbol Technologies, Inc., which is incorporated herein by reference in its entirety. Another conventional optical code is known as “MaxiCode”. It consists of a central finder pattern or bull's eye center and a grid of hexagons surrounding the central finder. It should be noted that the aspects of the inventions disclosed in this patent application are applicable to optical code readers, in general, without regard to the particular type of optical codes which they are adapted to read. The invention described may also be applicable to some associated image recognition or analysis.
Optical code readers may be laser-based or imager-based. In a conventional imager optical code reader an imaging engine is provided having an image sensor having an array of cells or photo sensors, such as a charge coupled device (CCD). In a linear imager reader the array may have one or two rows of photo sensors. In an area imager reader the array may be two-dimensional having two or more rows of photo sensors. The imaging engine further includes a lens assembly for focusing light incident on the image sensor and associated circuitry coupled to the image sensor outputting an image signal including an array of electrical signals corresponding to a field of view of the imager reader. The electrical signals are digitized and provided as image data to a processor for processing thereof, including processing the image data for decoding the optical code.
When originally introduced, the imager readers were placed directly in contact with the optical code being read. Since then the working range of many imager readers has increased, providing a working range comparable to some laser readers. However, unlike laser readers which are insensitive to motion, the performance of the imager reader has been limited due to sensitivity to motion inherent to the imaging technology used in the imager reader.
In order to eliminate or reduce motion, the imager reader needs to be held steady while scanning each optical code, which can be tedious, tiresome and/or reduce productivity. In applications in which rapid scanning is performed the need to rush may cause an increase in motion, which may cause smearing of the captured image and possible misreads, resulting in potential frustration and further reduction in productivity.
Similar to other camera systems, motion sensitivity can be reduced by reducing exposure time. However, a reduction of exposure time causes a reduction of the amount of light detected by the photo sensors which degrades signal quality of the electrical signals. Signal quality is known to vary with respect to distance between the imager reader (e.g., the photo sensors) and the target optical code being imaged, as well as with respect to contrast of the target optical code. Accordingly, an arbitrary reduction of exposure time could result in an in inadequate signal quality and an inability to decode the signal when scanning conditions are not favorable to good signal quality.
In commercially available imager readers variations in signal quality have been handled in different ways. In one commercially available imager reader, the exposure time is fixed to provide adequate signal quality at the furthest distance between the target and the imager reader at which the imager reader is intended to function. The electrical signals generated by the photo sensors are relatively strong when an optical code is positioned close to the imager reader when the internal illumination system of the imager reader is most intense. As the optical code is positioned further away from the from the imager reader, the intensity of the illumination system decreases as its light spreads out over a larger area. Observations of the signals generated by the photo sensors reveal that the signals amplitudes' decrease as the barcode is moved further away until the signal ultimately becomes too small for signal processing circuitry of the imager reader to handle, at which point the imager reader cannot read the optical code. The commercially available imager reader adapts to the variation in signal amplitude for allowing the imager reader to have a relatively large working range by using a signal processing circuit/digitizer that adapts to signal amplitude.
In another commercially available imager reader a variable gain amplifier is used that adjusts the gain for maintaining the signal amplitude within a desired range. As the distance between the imager reader and the target optical code increases the gain increases as well. The exposure time is fixed to provide adequate signal quality at the furthest distance at which the imager reader is intended to function. In both of the imager readers discussed above, the working range is greater than most users need, while the exposure time is fixedly set to provide adequate signal quality when the distance between the imager reader and the target is relatively large, even though a shorter exposure time would provide adequate signal quality at the most commonly used working distances and reduced sensitivity to motion.
Accordingly, it is an aspect of the present disclosure to provide a system and method for reducing the motion sensitivity of imager readers.